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    //! COAP encoder state used by CoAP encoding macros

    use cstr_core::CStr;      //  Import string utilities from `cstr_core` library: https://crates.io/crates/cstr_core
    use cty::*;               //  Import C types from cty library: https://crates.io/crates/cty
    use crate::{
        sys::console,
        encoding::tinycbor::CborEncoder,
        fill_zero, Strn, StrnRep,
    };

    /// Global instance that contains the current state of the CoAP encoder. Only 1 encoding task is supported at a time.
    pub static mut COAP_CONTEXT: CoapContext = fill_zero!(CoapContext);

    /// CoAP encoder state. Buffers the next key and value to be encoded.
    #[derive(Default)]
    pub struct CoapContext {
        /// Static buffer for the key to be encoded. Will be passed to Mynewt COAP encoder API.  Always null-terminated.
        key_buffer: [u8; COAP_KEY_SIZE],
        /// Static buffer for the string value to be encoded. Will be passed to Mynewt COAP encoder API.  Always null-terminated.
        value_buffer: [u8; COAP_VALUE_SIZE],
    }

    /// Size of the static key buffer
    const COAP_KEY_SIZE: usize = 32;
    /// Size of the static value buffer
    const COAP_VALUE_SIZE: usize = 32;

    /// Global CBOR root map
    static mut cbor_encoder0: CborEncoder = fill_zero!(CborEncoder);
    static mut cbor_encoder1: CborEncoder = fill_zero!(CborEncoder);

    impl CoapContext {

        pub fn json_set_text_string(&mut self, key: &Strn, value: &Strn) {
            let key_cstr: *const u8 =
                match key.rep {
                    StrnRep::ByteStr(bs) => { self.key_to_cstr(bs) }
                    StrnRep::CStr(cstr)  => { cstr }
                };
            let value_cstr: *const u8 =
                match value.rep {
                    StrnRep::ByteStr(bs) => { self.value_to_cstr(bs) }
                    StrnRep::CStr(cstr)  => { cstr }
                };
            unsafe {
                crate::libs::mynewt_rust::json_helper_set_text_string(
                    self.to_void_ptr(),
                    key_cstr as *const c_char,
                    value_cstr as *const c_char
                )
            };
        }

        /// Given a key `s`, return a `*char` pointer that is null-terminated. Used for encoding COAP keys.
        /// If `s` is null-terminated, return it as a pointer. Else copy `s` to the static buffer,
        /// append null and return the buffer as a pointer.
        pub fn key_to_cstr(&mut self, s: &[u8]) -> *const u8 {
            //  If null-terminated, return as pointer.
            if s.last() == Some(&0) { return s.as_ptr() as *const u8; }
            //  Else copy into static key buffer and return pointer to buffer.
            assert!(s.len() < COAP_KEY_SIZE, "big key");  //  Key too long
            self.key_buffer[..s.len()].copy_from_slice(s);
            self.key_buffer[s.len()] = 0;
            self.key_buffer.as_ptr() as *const u8
        }

        /// Given a value `s`, return a `*char` pointer that is null-terminated. Used for encoding COAP values.
        /// If `s` is null-terminated, return it as a pointer. Else copy `s` to the static buffer,
        /// append null and return the buffer as a pointer.
        pub fn value_to_cstr(&mut self, s: &[u8]) -> *const u8 {
            //  If null-terminated, return as pointer.
            if s.last() == Some(&0) { return s.as_ptr() as *const u8; }
            //  Else copy into static value buffer and return pointer to buffer.
            assert!(s.len() < COAP_VALUE_SIZE, "big value");  //  Value too long
            self.value_buffer[..s.len()].copy_from_slice(s);
            self.value_buffer[s.len()] = 0;
            self.value_buffer.as_ptr() as *const u8
        }

        /// Compute the byte length of the string in `s`.
        /// If `s` is null-terminated, return length of `s` - 1. Else return length of `s`.
        pub fn cstr_len(&self, s: &[u8]) -> usize {
            //  If null-terminated, return length - 1.
            if s.last() == Some(&0) { return s.len() - 1; }
            s.len()
        }

        /// Return the global CBOR encoder
        pub fn global_encoder(&self) -> *mut super::tinycbor::CborEncoder {
            unsafe { &mut super::g_encoder }
        }

        /// Create a new CBOR encoder for the current map or array, e.g. `key=root, suffix=_map`
        pub fn new_encoder(&self, key: &str, suffix: &str) -> *mut super::tinycbor::CborEncoder {
            console::print("new_encoder: "); console::print(key); console::print(suffix); console::print("\n");
            //  TODO: Allow multiple keys
            if (key, suffix)      == ("values", _ARRAY) { unsafe { &mut cbor_encoder0 } }
            else if (key, suffix) == ("values", _MAP)   { unsafe { &mut cbor_encoder1 } }
            else {
                assert!(false, "new_encoder fail");  //  TODO: No such encoder.
                unsafe { &mut super::root_map }
            }
        }

        /// Return the CBOR encoder for the current map or array, e.g. `key=root, suffix=_map`
        pub fn encoder(&self, key: &str, suffix: &str) -> *mut super::tinycbor::CborEncoder {
            console::print("encoder: "); console::print(key); console::print(suffix); console::print("\n");
            //  TODO: Allow multiple keys
            if (key, suffix)      == ("root", "_map")   { unsafe { &mut super::root_map } }
            else if (key, suffix) == ("values", _ARRAY) { unsafe { &mut cbor_encoder0 } }
            else if (key, suffix) == ("values", _MAP)   { unsafe { &mut cbor_encoder1 } }
            else {
                assert!(false, "encoder fail");  //  TODO: No such encoder.
                unsafe { &mut super::root_map }
            }
        }

        /// Fail the encoding with an error if `res` is non-zero.
        pub fn check_result(&self, res: u32) {
            assert_eq!(res, 0, "enc fail");
        }

        /// Fail the encoding with an error
        pub fn fail(&mut self, err: CoapError) {
            assert_eq!(err, CoapError::OK, "enc fail");
        }

        /// Cast itself as a `*mut c_void`
        pub fn to_void_ptr(&mut self) -> *mut c_void {
            let ptr: *mut CoapContext = self;
            ptr as *mut c_void
        }
    }

    /// Error codes for COAP encoding failure
    #[derive(Debug, PartialEq)]
    pub enum CoapError {
        /// No error
        OK = 0,
        /// Encoded value is not unsigned integer
        VALUE_NOT_UINT = 1,
    }

    /// Convert the type to array of bytes that may or may not end with null
    pub trait ToBytesOptionalNull {
        /// Convert the type to array of bytes that may or may not end with null
        fn to_bytes_optional_nul(&self) -> &[u8];
    }

    /// Convert the type to array of bytes that may or may not end with null
    impl ToBytesOptionalNull for [u8] {
        /// Convert the type to array of bytes that may or may not end with null
        fn to_bytes_optional_nul(&self) -> &[u8] {
            self
        }
    }

    /// Convert the type to array of bytes that may or may not end with null
    impl ToBytesOptionalNull for str {
        /// Convert the type to array of bytes that may or may not end with null
        fn to_bytes_optional_nul(&self) -> &[u8] {
            self.as_bytes()
        }
    }

    /// Convert the type to array of bytes that may or may not end with null
    impl ToBytesOptionalNull for &str {
        /// Convert the type to array of bytes that may or may not end with null
        fn to_bytes_optional_nul(&self) -> &[u8] {
            self.as_bytes()
        }
    }

    /// Convert the type to array of bytes that may or may not end with null. CStr always ends with null.
    impl ToBytesOptionalNull for CStr {
        /// Convert the type to array of bytes that may or may not end with null. CStr always ends with null.
        fn to_bytes_optional_nul(&self) -> &[u8] {
            self.to_bytes_with_nul()
        }
    }

    /*
    /// Convert the type to array of bytes that may or may not end with null. Strn always ends with null.
    impl ToBytesOptionalNull for crate::Strn {
        /// Convert the type to array of bytes that may or may not end with null. Strn always ends with null.
        fn to_bytes_optional_nul(&self) -> &[u8] {
            if self.cstr != 0 as *const u8 {
                return unsafe { ::core::mem::transmute::<*const u8, &[u8]>(self.cstr) };
                //return self.cstr as &[u8];
            }
            self.bytestr
        }
    }
    */

    /// Root of CoAP document
    pub const _ROOT: &str = "root";

    /// Map element of CoAP document
    pub const _MAP: &str = "_map";

    /// Array element of CoAP document
    pub const _ARRAY: &str = "_array";

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